Functional Differences in Water‐Use Patterns of Contrasting Life Forms in Great Basin Steppelands

Abstract

The temporal patterns of evapotranspiration were monitored for 2 yr for four species of differing life form that currently form near monoculture communities in the Great Basin, USA, a region with a growing season spanning early spring to autumn and predictable overwinter water accumulation in the vadose zone. Species included an annual grass (Bromus tectorum L.), a perennial grass [Agropyron desertorum (Fisch. ex Link) Schult.], a shrub (Artemisia tridentata Nutt. ssp. wyomingensis Beetle and Young), and a tree [Juniperus osteosperma (Torr.) Little]. The two grasses and shrub were growing on the same soil type with uniform texture and subject to near surface percolation of the vadose zone only, while J. osteosperma was growing on soils with a petrocalcic layer below which water was near field capacity. These monotypic stands were found to differ in quantity and timing of vadose zone water use, in use pattern of shallow and deeper water resource pools, and in depth and quantity of rainwater hydraulically redistributed. All species rapidly utilized shallow vadose zone water in the spring when growth was observed, but use of deep vadose zone water varied by life form and was not linked with the period of growth for any species. Water in the vadose zone of the grass species increased between years, with evapotranspiration less than precipitation inputs and contrasted to water use in A. tridentata where water use approximately equaled precipitation inputs. Juniperus osteosperma used water below the petrocalcic zone, particularly in late summer. Water use by all species was consistent with the concept of a shallow vadose zone “growth pool” of water and a deeper vadose zone “maintenance pool” used during the summer drought period. The patterns of water use suggest that water per se is not a limited resource for survival, but influences the availability of nutrients necessary for plant growth that are associated with shallow vadose zone water. We postulate that cold‐adapted plants in the Great Basin have converged on a general pattern of rapidly utilizing soil moisture in shallow depths, in part, to influence nutrient availability. Our results strongly suggest describing the pool dynamics of vadose zone water will be necessary to further our understanding of plant fitness, interactions among species for resources, and species coexistence in arid and semiarid ecosystems.